Device and method for calibrating a weighing apparatus

ABSTRACT

A system for calibrating a plurality of weighing installations of the kind having a working configuration of at least one working load cell ( 14 ) with a fixed side and a load-bearing sides, a support ( 16 ) for the fixed side, and a load ( 12 ) operatively mounted on the load-bearing side, comprises portable apparatus moveable between installations to be calibrated, and fixed apparatus at each installation. The portable apparatus includes a reference load cell ( 30 ) and a fluid ram ( 32 ), the fixed apparatus includes anchorage means ( 22 ) fast with the working load cell support ( 16 ), and the reference cell and the ram are removably connectable between the anchorage means and the load-bearing side of the working load cell to apply calibrating loads to the working load cell in its working configuration. As many rams and reference cells may be provided as there are working load cells for any weighing installation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to calibrating weighing installations.

2. Description of Related Art

Installations such as weigh platforms or vessels (such as silos, tanksand hoppers) which are used in industrial processes for information andprocess control purposes usually have a working configuration of atleast one working load cell, a support for a fixed side of the loadcell, and a load mounted on a load-bearing side of the load cell. Theload is usually transmitted to the working load cell through a vesselsupport bracket or other special bearing surface provided on the vessel,adapted to carry its weight and the weight of any contents. Typically,three or four working load cells, with corresponding supports and loads,are used in any one installation, so the aggregate load is the sum ofall the separately measured loads. Some platforms and vessels use amechanical arm system to transfer their weight to a single load cell.The term vessel will be used herein for convenience to include allreceptacles, containers, platforms or the like for supporting whateveris to be weighed by a weighing installation.

Such installations need to be calibrated and re-calibrated from time totime, because of drift or creep in the electrical or mechanicalproperties of the load cells, or in the tare of the weigh platform orvessel. It is not usually practicable or good practice to remove theload cell or cells from the installation for re-calibration, because ofthe substantial impact this will make on the availability of theinstallation. In an industrial process, it normally would meanproduction downtime. Instead, the usual method adopted is to load theinstallation with a standard weight, which is compared with the totalweight indicated by the working load cells. This indicated total weightis conventionally the single integrated or summated output of all loadcells combined. Conventional weighing calibration is the calibration ofthe whole system, including all load cells, sources and measures ofapplied excitation voltage to the load cells, and meters forinterpreting the corresponding load cell output signals. A necessaryconsequence is that, if any part of this system fails or is adjusted orreplaced, the whole system must be recalibrated.

The use of standard weights can be time-consuming, dangerous andinaccurate. Considering that this invention is particularly devised foruse with weight vessels typically ranging from 250 kg to over 100,000 kgcapacity, the limitations of using dead weights of known value,especially at the higher end of this range, will be clear. In the caseof tanks, metered quantities of water can be used, calculating weightsfrom the known volumes and density, but it is known that flow meters areof limited accuracy. Loading a vessel with known weights may not bepossible over the full capacity of the vessel, so that calibration ofthe maximum load is dependent upon extrapolation from lower loads, butthe calibration curve may not in fact be linear, uniform or regular.

However, although such methods are conventionally used, they still implya substantial interruption in the normal operation of the weighinginstallation, which can be especially costly if it is normally in use ina continuous industrial process.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide an alternative system andmethod for calibrating weighing installations of the above kind, whichenable calibration to be carried out quickly and accurately.

The present invention concerns aspects of the system for calibratingweighing installations described below. The scope of the inventionextends to all novel aspects of the system, including methods of puttingit into effect, whether individually or in combination with any of theother features disclosed herein.

More specifically, in one aspect of the invention, a system forcalibrating weighing installations of the aforementioned kind comprisesportable apparatus movable between installations to be calibrated, andfixed apparatus at each installation to be calibrated, wherein theportable apparatus includes a reference load cell and a fluid ram, thefixed apparatus includes anchorage means fast with the working load cellsupport, and the reference cell and the ram are removably connectablebetween the anchorage means and the load-bearing side of the workingload cell to apply calibrating loads to the working load cell in itsworking configuration.

The fixed elements of the calibration system need be no more than theanchorage means only, at each working load cell support. The workingload cells, which generally tend to be shear beams of one design orother, normally rest on load plates. The anchorage means can withadvantage be incorporated into each load plate. Alternatively, theanchorage means can be provided on the solid base to which the loadplates themselves are fastened. The anchorage may be mechanical, orelectromagnetic, in which case a ferromagnetic material may be providedas the anchorage means, to be engaged by electromagnets in the portableapparatus.

The anchorage means are desirably close to the working load cells. It isin practice unlikely that the calibrating loads will be applied directlyto the working load cell, because its load-bearing side is alreadycarrying the weigh vessel. The calibrating load is accordingly appliedto the weigh vessel itself, and it is generally convenient to applythose loads to the parts of the structure that are designed to take andtransmit loads, which will normally be adjacent the point at which theweigh vessel is mounted on the working load cell, for example at anexisting mounting bracket.

The portable elements in the system include the fluid ram, normally ahydraulic ram, and the reference load cell, and may also includesupplementary fixings, for example a cradle to fasten to the anchoragemeans and hold the ram and reference cell in a proper position to exertcalibrating loads on to the working load cell. Preferably, positionadjustment means are included in the portable elements, to permit thereference cell to be properly positioned in relation to the working cellin different weighing installations, in which the anchorage means maynot be identically positioned in relation to the working cells.

The reference cell should be accurate and may be periodicallystandardised by secondary referents traceable to an appropriate ultimatestandard.

In a method in accordance with the invention, corresponding to the useof the system described above, the reference cells and the ram areconnected between the anchorage means and the load-bearing side of theworking load cell of each installation in turn, and calibrating loadsare applied to each working load cell in its working configuration.

In the common case where a weighing installation has more than oneworking load cell, the portable apparatus may have as many referenceload cells and fluid rams as are necessary to apply calibrating loads toeach one of the plurality of working load cells in its workingconfiguration—usually simply one reference cell and one ram for eachworking load cell. This enables faster calibration. It also enablescalibration of each load cell selectively as the sole cell to which aload is applied, or while other loads are applied to other cells, or, ina more natural working condition, while similar loads are applied to theother cells, so that the loads are balanced between the working cells.Accordingly, the portable apparatus may include means for supplyingfluid under pressure to the fluid ram associated with each one of theplurality of reference load cells, means for recording each calibratingload applied thereto as measured by the reference load cells, and meansfor recording the corresponding output of the working load cells toprovide a calibration record. The method of the invention may be appliedaccordingly,

Likewise, the system of the invention may include control means forvarying the pressure of the fluid supplied to the rams whereby tocontrol the force exerted by a given ram on the corresponding referenceand working load cells in a sequence of calibration steps for eachworking load cell, and may include switch means for diverting fluid tothe ram or rams associated with each of the plurality of reference loadcells in turn, and for selecting the outputs of the corresponding loadcells for recording, with corresponding applications of the method ofthe invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Embodiments of the invention are illustrated, by way of example only, inthe accompanying drawings, in which:

FIG. 1 is a front elevation of a system in accordance with theinvention, with a cross beam shown in section;

FIG. 2 is a side elevation of the system, one face of a weigh vesselsupport bracket having been cut away for clarity;

FIG. 3 is a front elevation of a modification of the system shown inclaim 1, with a different anchorage;

FIG. 4 is a diagram illustrating the use of the invention with amulti-cell weighing installation; and

FIG. 5 is a diagram illustrating the application of the invention to theconventional elements of a multi-cell weighing installation.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 of the drawings show the system of the invention asapplied at one weighing station of a permanent weighing installation formeasuring loads carried by a vessel 10, provided with, typically, threeor four such weighing stations. At each weighing station the vessel issupported by a rigid bracket 12, which rests on a cantilever shear beamworking load cell 14. This in turn is mounted on a load plate 16, boltedto a concrete beam 18 which is part of the permanent structure carryingthe weigh vessel.

In accordance with the invention, anchorage means are provided which arefast with the working load cell support. The anchorage means comprisetwo pairs of parallel upstanding steel webs 22 welded to the load plate16 on either side of the load cell 14. The webs are pierced with slots24.

The pierced webs provide an anchorage that is fast with the working loadcell support. Similar anchorages are provided in association with eachworking load cell, so that the portable apparatus, described below, canbe used to calibrate each working load cell in turn.

The principal elements of the portable apparatus are a reference loadcell 30, and a hydraulic ram 32. The ram presses the reference load celldownwardly against bracket 12 supported on working load cell 14. Inorder to do this, a cradle 34 is provided. The cradle includes typicallytwo tie bars 36 joined by a cross beam 38. For greater loadingcapability, additional tie bars may be used, eg four in total. Each tiebar extends upwardly from a base 40 which is retained between arespective one of the pairs of anchorage webs 22 by means of bolts orpins 42 in the slots 24. The upper ends of tie bars 36 are retained inslots 44 in the cross beam. A vertical hole 46 through the centre of thecross beam provides an attachment point for ram 32. By virtue of theslots 24 and 44, which extend at right angles to one another, thehorizontal position of the ram can be adjusted until it is exactly overthe load bearing region of load cell 14.

A spherical male/female self-levelling washer combination 48 is locatedbetween the reference load cell 30 and the upper surface of bracket 12against which it will bear, to accommodate any small misalignmentbetween the portable apparatus and the surface of the bracket. It may bebeneficial to incorporate a level bubble in cross beam 38, to assist insetting up the portable apparatus with ram 32 vertical.

The reference load cell 30 is suitably a pancake load cell, in which acentral core, over combination washer 48, is supported by shear websfrom an outer rim, which is acted upon by ram 32.

An adjusting screw 50 is provided to give fine adjustment to thevertical position of reference load cell 30. Desirably, the screw isused to back the reference cell off from bracket 12 in order to provethe zero load, with the vessel empty.

It can thus be seen that the loadings applied to working load cells 14by hydraulic ram 32, held down by tie bars 36 anchored to load plate 16,can be accurately measured by reference cell 30 and used to calibrateworking cell 14.

As illustrated, the cradle 34 is part of the portable apparatus.However, if fixed in position at each working load cell location, acorresponding cradle could be provided as part of the fixed apparatus,for example if the portable apparatus were required to be used in avariety of locations in which a single cradle design would not besufficiently adaptable. Furthermore, in some weigh vessel designs, thebracket 12 could be configured in relation to the fixed vessel supportstructure that the ram and reference cell could act between theanchorage means and the working cell without the need for anyintermediate connecting member such as the cradle. In effect, the cradleitself then provides the anchorage means fast with the working load cellsupport. The relative positions of the reference load cell and the ramcan of course be varied to adapt to the physical space available.

FIG. 3 shows a modification in which the tie bars 36 are anchored toload plate 16 by electromagnetic means. These comprise flat facedelectromagnets 54, which when energised grip flat areas 56 of the loadplate 16. At least the flat areas 56 are accordingly of ferromagneticmaterial to ensure sufficient magnetic coupling. The respective flatareas could be of other mutually complementary conformations ifcircumstances required it.

As a matter of economics, the fixed parts of the system, which arereplicated at each working load cell with which the system is used,should use the least costly components, while the more costly componentsshould as far as practicable be incorporated into the portableapparatus.

By providing a portable ram and reference cell, and pre-installedanchorage points at each working load cell location, weighinginstallations can be calibrated quickly and economically with minimalinterference and disturbance to production weighing installations.

In a modification of the invention, irrespective of the presence of anyworking load cell, the portable apparatus of the system can be used toapply lifting force between anchorage means and a vessel in order to actas portable weighing apparatus, for intermittent or periodic weighingoperations.

By applying a load through a reference cell directly to the working loadcell or cells or vessel, it is not necessary to utilise pulleys,bearings or levers that introduce mechanical errors into the accuracy ofthe calibration, and the calibration apparatus can be made portable, asdescribed, between vessels of different sizes and designs.

FIGS. 4 and 5 illustrate the use of a system as illustrated in FIGS. 1and 2 or FIG. 3.

A weighing vessel (not shown) is supported on three working load cells14 spaced equally about its circumference, and controlled forces areapplied to each load cell 14 by one hydraulic ram 32 acting through onereference load cell 30 at each cell 14, for example as described withreference to FIGS. 1 to 3.

The portable elements of the system, besides the three pairings of a ram32 and a reference cell 30, comprise equipment carried by a mobilewheeled trolley 60, and connecting lines 64 and 78.

Trolley 60 carries a hydraulic power unit 62, which includes aconventional hydraulic pump and reservoir, connected by hydraulic hoses64 to the respective rams 32 for applying calibrating loads to therespective working load cells in their working configurations. Thetrolley includes a computer 66, which is provided with simple controlsoftware to enable an operator to input commands and observe and recordthe outcomes, at a conventional keyboard and screen and disk drive orother input and output and storage devices as may be found appropriatefor any given case. In particular, computer 66 records each calibratingload applied to the reference load cells as measured by them, andrecords the corresponding outputs of the working load cells. This lastmay be input manually, from an operative's observation of a display ofthe measured load, or directly, by a suitable connection (such as aserial data connection) to the weighing indicator 84, see below withreference to FIG. 5. Computer 66 is operatively connected both to thehydraulic power unit and an instrument housing 70. The hydraulic powerunit includes switch means for diverting fluid to the rams 32 in turn,and is controlled by the computer 66 to vary the pressure of the fluidsupplied to the rams to control their force in a sequence of calibrationsteps for each working load cell. The instrument housing 70 contains anelectrical power supply 72, a digital volt meter 74 and a channelscanner 76. The channel scanner is a switch that distributes excitationvoltages to the respective reference load cells 30, and performs dataacquisition from those cells, through multicore electrical cables 78, insynchrony with the operation of the rams 32.

Although FIG. 4 shows three reference cells to be calibrated, theequipment carried by trolley 60 is sufficient to supply hydraulic fluidto up to eight rams, and to communicate through up to eight channels,corresponding to up to eight working load cells in a weighinginstallation.

Each working load cell has its normal input/output multicore cable 80.These are part of the permanent weighing installation at this site, andconduct the excitation voltage to, and output signal from, each workingload cell. The respective cables 80 are combined at junction box 82,which is connected by a single multicore cable 83 to weighing indicator84. This is a standard item of equipment and comprises in particular adisplay screen 86 and a number of manual controls 88. It is providedwith a power supply 90, and communicates with one or more additionaloutput devices 92, which may include control room and other slavedisplays and data supply for other data users, in particular may includedata for the computer 66 for use during calibration, and data for othercontrol and recording functions when the weighing installation is partof an industrial production process.

It will be seen that all the permanent and fixed elements of theweighing system are shown in FIG. 5, and all the portable elements areshown in FIG. 4. Essentially, in accordance with the invention, one setof the portable elements can be used with numerous different fixedweighing installations. After calibration is completed, the hoses 64 andcables 78 are disconnected and the rams 32 and reference cells 30 areremoved from the anchorage means adjacent each reference load cell 14.These can be stored for convenience in trolley 60, which can then bemoved to another site, and the apparatus reassembled.

The actual calibration process can be carried out in a number of ways.In a typical case, the weigh vessel is first emptied, and all readingsare set to zero. Then, pressure is applied through hoses 64 to each ram32 in turn, preferably to a convenient round figure force exertedthrough reference cell 30. Examples might be steps of 250 kg up to 3,000kg, according to the capacity of the vessel and the working load cells.Obviously, the reference cells will be chosen to have an appropriateoperative range. As the loads are imposed on each working load cell inturn, channel scanner 76 ensures an appropriate excitation voltagesupply to the reference load cell 30, and digital volt meter 74 recordsthe corresponding output, which is interpreted as a force by computer66. This is possible because computer 66 has been loaded with allrelevant data for the individual reference load cells 30, which willhave been standardised against a prime standard at periodical intervals.Likewise, the applied voltage and the output voltage will be measured byperiodically standardised equipment.

In this way, a calibration curve can be built up for each working loadcell 14, by comparing the applied load as determined by the portableapparatus with the indicated load shown on display screen 86. It isrecommended to check the working load cells individually in order todetermine whether any is defective. This would not show up on aconventional whole system calibration, but it can adversely affectaccuracy if a solid load on the weigh vessel is off centre. Thecalibration can be repeated as often as required, in order to produce anaverage. The final calibration is carried out loading all working cellssimultaneously, in order to provide the overall calibration of theinstallation.

Manual controls 88, which are provided as standard on typical weighingindicators 84, enable the indicator to be reset to display weights fromthe weigh vessel that are within an acceptable tolerance from theaccurately determined weights known to be applied by the portableapparatus. After any adjustment of the response of the weighingindicator 84, it is permissible to run the calibration cycle again as atest of accuracy.

It will be appreciated that the system can equally be applied to theworking load cells individually, or simultaneously, by simple adjustmentto the controls. A calibration certificate can be produced, includinginformation such as linearity, hysteresis, best fit straight line andrepeatability. We have found that accuracies of within 1 part in 5000are possible, using measuring elements all traceable to a primarystandard source. Further, in contrast to the use of dead weights,calibration forces can be applied up to the full scale of the capacityof the weighing vessel.

Furthermore, in a weighing installation calibrated by means of thepresent invention, it is possible to replace individual reference loadcells by other load cells previously calibrated, either at the samecapacity or of a different capacity, and retain a fully traceablecalibration system without the need to recalibrate the whole systemafter replacement of the reference load cells. In this way, working loadcells giving a weigh vessel a full scale loading of say 10 tonnes can becalibrated by one set of reference load cells, and a weigh vessel with afull scale loading capacity of say 50 tonnes can then be calibratedwithout the need to recalibrate the whole system. Any defective part ofthe calibration system can be simply replaced by another similar partthat has itself been tested against a standard.

It has been estimated that to calibrate ten conventional weigh vesselsusing dead weights, with one full scale calibration per vessel, mightrequire the provision, movement and storage of the dead weights, threeoperations, and 120 man-hours of labour. In contrast, by using thesystems and methods of this invention, the calibration can be completedby one operative, in 8 man-hours, achieving three full scalecalibrations per vessel.

What is claimed is:
 1. A system for calibrating a plurality of weighinginstallations of the kind having a working configuration of a pluralityof working load cells each with a fixed side and a load-bearing side,supports for the fixed sides of the load cells, and a load operativelymounted on and distributed between the load-bearing sides of the loadcells, comprising portable apparatus moveable between installations tobe calibrated, and fixed apparatus at each installation to becalibrated, wherein, the fixed apparatus at each installation includes aplurality of anchorage means fast with each working load cell support,the portable apparatus includes as many reference load cells and fluidrams as are necessary to apply calibrating loads simultaneously to eachof the plurality of working load cells in the working configuration ofany of said installations, a source of fluid under pressure, and meansfor supplying fluid under pressure from the source simultaneously to thefluid rams associated with each one of the plurality of reference loadcells, and the reference cells and the rams are removably connectablebetween the anchorage means and the load-bearing sides of the workingload cells to apply calibrating loads to the working load cells in theworking configurations of the installations, wherein the system isoperable to calibrate each load cell selectively as well assimultaneously and, the portable apparatus includes switch means fordiverting fluid to the ram or rams associated with each of the pluralityof reference load cells in turn, and for selecting the outputs of thecorresponding load cells for recording.
 2. A system according to claim 1wherein the support for the fixed side of the load cell comprises a loadplate, and the anchorage means are incorporated into each load plate. 3.A system according to claim 1 wherein the support for the fixed side ofthe load cell comprises a load plate fastened to a solid base, and theanchorage means are provided on the solid base.
 4. A system according toclaim 1 wherein the anchorage means comprise two pairs of parallelupstanding webs on either side of the load cell, the webs having slotsto provide an anchorage.
 5. A system according to claim 1 wherein theanchorage means comprise flat areas of ferromagnetic material engageableby electromagnets carried by the portable apparatus.
 6. A systemaccording to claim 1 wherein the portable apparatus includes a cradleremovably connectable to the anchorage means, holding the fluid ram inposition to apply a calibrating load to the working load cell.
 7. Asystem according to claim 6 wherein the cradle includes tie bars forengagement with the anchorage means joined by a cross beam to which theram is attached.
 8. A system according to claim 6 wherein the cradleincludes position adjustment means to permit the reference cell to beproperly positioned in relation to the working cell.
 9. A systemaccording to claim 1 wherein the calibrating load is transmitted to theworking load cell through a part of a weigh vessel.
 10. A systemaccording to claim 9 in which the calibrating load is transmitted to theworking load cell through a vessel support bracket.
 11. A systemaccording to claim 1 including a self levelling washer located betweenthe portable apparatus and the load-bearing side of the working loadcell.
 12. A system according to claim 1 wherein the reference load cellis a pancake load cell in which a central core is supported by shearwebs from an outer rim.
 13. A system according to claim 1 wherein theportable apparatus includes means for recording each calibrating loadapplied to the working load calls as measured by the reference loadcells, and means for recording the corresponding output of the workingload cells to provide a calibration record.
 14. A system according toclaim 1 wherein the portable apparatus includes control means forvarying the pressure of the fluid supplied to the rams whereby tocontrol the force exerted by a given ram on the corresponding referenceand working load cells in a sequence of calibration steps for eachworking load cell.
 15. A method of calibrating a plurality of weighinginstallations of the kind having a working configuration of a pluralityof working load cells each with a fixed side and a load-bearing side,supports for the fixed sides of the load cells, and a load operativelymounted on and distributed between the load-bearing sides of the loadcells, comprising providing portable apparatus moveable betweeninstallations to be calibrated, and providing fixed apparatus at eachinstallation to be calibrated, wherein the fixed apparatus at eachinstallation includes a plurality of anchorage means fast with eachworking load cell support; and the portable apparatus includes as manyreference load cells and fluid rams as necessary to apply calibratingloads simultaneously to each of the plurality of working load cells inthe working configuration of any of said installations, and at eachinstallation in turn, connecting the reference cells and the ramsbetween the anchorage means and the load-bearing sides of the workingload cells, and supplying fluid under pressure from a source on theportable apparatus to the fluid rams associated with each of theplurality of reference load cells whereby simultaneously to applycalibrating loads to each working load cell in the working configurationof the installation; supplying fluid from the source to the ramsassociated with the reference load cells selectively as well assimultaneously when applying the calibrating loads to each working loadcell at an installation and recording each calibrating load applied tothe working load cells as measured by the reference load cells,recording the corresponding output of the working load cells to providea calibration record and diverting fluid to the ram or rams associatedwith each of the plurality of reference load cells in turn, andselecting the outputs of the corresponding load cells for recording. 16.A method according to claim 15 comprising varying the pressure of thefluid supplied to the rams whereby to control the force exerted by agiven ram on the corresponding reference and working load cells in asequence of calibration steps for each working load cell.
 17. A methodaccording to claim 15 comprising the step of providing switch means fordiverting fluid to the ram or rams.